1. Field of the Invention
The invention relates generally to the field of analog to digital sigma-delta signal modulation.
2. Discussion of the Related Art
Discrete-time (DT) sigma-delta modulators have been successfully implemented using switched-capacitor techniques during the past decade. In a switched-capacitor implementation of sigma-delta modulators, integrators are required to settle with an adequately small error at the end of each clock phase. This calls for fast amplifiers and increased power dissipation. A second drawback of the switched-capacitor approach is aliasing of thermal noise and consequently increased in-band noise. Continuous-time (CT) sigma-delta modulation is an alternative way of converting analog signals to digital without the above-mentioned drawbacks. In the CT-modulator, all integrators operate in the continuous-time domain and sampling occurs at the same time as quantization. As a consequence, CT-modulators are less demanding in terms of biasing current. Moreover, they provide an anti-aliasing filter without cost.
In the current state of the semiconductor technology nearly all analog integrated circuits are realized using fully-differential structures in order to improve immunity to noise and device inaccuracies. Sigma-delta modulators which have become an indispensable part of most mixed-signal circuits are required to be immune to the large amount of noise inherent to the switching environment of this class of circuits. Furthermore, differential structures offer 3dB of signal-to-thermal noise improvement.
However, these advantages are gained only at the price of additional circuitry necessary to control the output common-mode voltage of a differential structure. On the other hand, this portion of the circuit is desired not to affect the processing of the input signal. Since the common-mode feedback (CMFB) circuit cannot be designed independently from the differential portion of the circuit, optimum design of fully-differential structures is usually subject to many trade-offs.
There is a need for the following embodiments. Of course, the invention is not limited to these embodiments.
In accordance with one aspect of the invention, a method for continuous-time sigma-delta modulation of an input signal includes, calculating an integrator input signal as a difference between the input signal and a modulation feedback signal, continuous time integrating the integrator input signal to produce an integrator output signal having a common mode voltage, determining the common mode voltage of the integrator output signal using a discrete-time process, determining an integrator feedback signal as a function of the common-mode voltage and using the integrator feedback signal to maintain the common mode voltage at a substantially constant value, sampling and quantizing the integrator output signal to produce a sigma-delta modulated output signal, and converting the sigma-delta modulated output signal from a digital signal to an analog signal, to produce the modulation feedback signal.
In accordance with another aspect of the invention, an apparatus for continuous time sigma-delta modulation, includes an adder to calculate an integrator input signal as a difference between an input signal and a modulation feedback signal, a continuous time integrator to integrate the integrator input signal to produce an integrator output signal having a common mode portion, a discrete-time common mode feedback circuit which produces an integrator feedback signal as a function of the common mode portion of the integrator output signal, the feedback signal being used to maintain the common mode portion at a substantially constant value, a quantizer coupled to receive the integrator output signal and to produce a quantized output signal; and a digital-to-analog converter coupled to receive the quantized output signal to produce the modulation feedback signal. The continuous time integrator may be an active RC integrator or an active GmC integrator.
In accordance with yet another aspect of the invention, an apparatus for continuous time sigma-delta modulation includes a continuous time integrator to integrate an input signal to produce an integrator output signal having a common mode component, and a discrete-time common mode feedback circuit which produces an integrator feedback signal as a function of the common mode component of the integrator output signal, the integrator feedback signal being used by the continuous time integrator to maintain the common mode component at a substantially constant value. The continuous time integrator may be an active RC integrator or an active GmC integrator.
In accordance with another aspect of the invention, an electronic apparatus includes a continuous time integrator to integrate an integrator input signal to produce an integrator output signal having a common mode portion, and a discrete-time common mode feedback circuit which produces an integrator feedback signal as a function of the common mode portion of the integrator output signal, the integrator feedback signal being used by the continuous time integrator to maintain the common mode portion at a substantially constant value.